Less than 24 hours after the FBI posted photos of two suspects in Monday’s Boston Marathon bombings, the world has learned an astounding amount, with astounding speed, about the two men — one of whom is now dead after a murder, car chase and shootout that climaxed in Watertown, Mass., last night. The other suspect, his brother, remains at large. But for all the detail emerging today, a huge question remains unanswered: What motivated their alleged actions?

The dead man is 26-year-old Tamerlan Tsarnaev. Still on the loose is his younger brother Dzhokhar Tsarnaev, who is just 19. Media accounts say the men were born abroad but have been in America for a decade or more.

There are conflicting accounts of where they lived before arriving in the U.S., though all suggest roots around the Caucasus region of southern Russia. By some accounts, the family moved to the U.S. about a decade ago to escape brutal violence in the Russian republic of Chechnya. The English-language Russian news outlet RT reports that in 2001 the family moved from the central Asian country of Kyrgyzstan to the Russian republic of Dagestan; but according to his page on a Russian social networking site, Vkontakte, the younger brother, Dzokhar, attended school in Dagestan from 1999 to 2001. The Associated Press found their father today in Dagestan’s capital, Makhachkala. Dagestan lies at Russia’s southern extremity, not far from Chechnya.

After moving to the U.S., both men grew up on Norfolk Street in Cambridge, Mass., not far from the main campus of Harvard University and the Massachusetts Institute of Technology, where a campus police officer was shot to death last night. Neighbors interviewed by TIME in the ethnically mixed, transitional neighborhood said the boys used to play soccer in the street and the father worked on his cars behind the family house.

Online glimpses of Tamerlan suggest a young man who never assimilated to his host country — “I don’t have a single American friend,” he is quoted as saying in this photo essay.

By contrast, however, Dzhokhar appears to have been a happy, well-adjusted and rather normal teenager, evident in part from his social-media footprint. More ominously, he also apparently expressed interest in Islamic political causes, including the armed Syrian rebellion and the cause of Chechen independence.

Authorities are undoubtedly focused on assessing the significance of hints the men may have had ties to radical Islam and the Chechen-independence movement. Chechnya may be best known in America for its violent separatist battles with Moscow (involving some of the most dreadful terrorism of recent years).

But the Islamic radicalism breedingviolently in Russia’s Caucusus region has inspired Chechens to join jihadi causes elsewhere in the world, possibly including fighting alongside the Taliban in Afghanistan. (This report suggests that no Chechens have been among the detainees at the U.S. military‘s Guantánamo Bay prison camp, though a human-rights lawyer who follows Gitmo cases cautions the report is not exhaustive.) Just last month, French and Spanish authorities arrested three Chechens in Paris for planning terrorist attacks.

None of this proves a motive for the attacks. What authorities don’t know is whether the brothers, if guilty, were influenced by radical religious beliefs or acted out of some petty grievances — that they simply were, as their furious uncle said of Tamerlan this morning, losers. Although there have been multiple religiously motivated terroristplots in the U.S. in recent years, the recent history of mass murder in America is a story of mental illness and social isolation, not religious fundamentalism.

For now, counterterrorism officials will have to assume the worst. Beyond capturing Dzhokhar Tsarnaev, their top priority will be determining whether a larger terrorist organization may be at work here, one that could be plotting further attacks.

The original version of this article has been updated to include reports that Dzhokhar Tsarnaev attended school in Dagestan from 1999 to 2001.